1. Field Of The Invention
The present invention relates in general to a transmission and reception protocol and, in particular, to a method of communicating between two or more radio transmitter/receivers utilizing a first pseudo-random cycle for synchronization and a second pseudo-random cycle for communication.
2. Background Art
Some radio transmitter/receivers, such as some cordless telephones, citizen band radios and the like, utilize spread spectrum technology. One type of spread spectrum communication is called frequency hopping, wherein each associated radio transmitter/receiver hops from channel to channel in a psuedo random sequence. The benefits of spreading transmission among numerous channels in the available spectrum include evenly utilizing the available spectrum and avoiding "fast fading" concerns.
Given the proliferation of this type of radio, particularly cordless telephones, an infinite number of channels is desirable to achieve minimal interference. However, due to regulations, available spectrum and radio characteristics, this number is relatively restricted. Within this relatively restricted number of channels it would be optimal to select each channel in a truly random fashion. However, it is clear that if selection were truly random it would not be practical to synchronize the channel selection at the receiver and the transmitter, hence some form of pseudo-random channel selection is used.
It is a characteristic of pseudo-random sequences that after a finite number of selections the sequence will repeat. In order to approximate a truly random sequence the repeat period, or "cycle," of the pseudo-random sequence needs to be very long. The result of all these constraints has been radio transmitter/receivers which hop over typically dozens of channels using pseudo-random sequences with very long cycle times.
Where pseudo-random cycle times are very long, synchronization of two radio transmitter/receivers becomes difficult. One approach has been to provide additional information, such as a small number of designated control channels upon which a "master" radio always transmits synchronization information to control a "slave" radio. Thus, an unsynchronized slave radio merely needs to sit and wait on a control channel for the master radio to hop onto the channel and transmit the synchronization information. This approach assumes that communication is continuous and only the master initiates communications. However, particularly in the case of cordless telephones, these assumptions are invalid. In fact, cordless telephones communicate at irregular, infrequent intervals and communication may be initiated by either the master (base unit) or the slave (handset).
It is thus an object of the present invention to provide a quick method of establishing synchronization between a master radio transmitter/receiver and one or more slave radio transmitter/receivers without the need for extended transmission periods by the master radio transmitter/receiver.
It is an associated object of the present invention to provide two types of pseudo random codes for generating the hopping cycles, wherein the one type of code that is used while the radios are idle--and only intermittently transmitting--is much shorter than the other code, which is utilized during continuous communications to achieve the full benefit of spread spectrum technology.
These and other objects of the invention will become apparent in light of the attached specification and claims.